Propargyl bromide (3-bromopropyne) is known to be useful as a soil fumigant for control of fungi, nematodes, and undesirable plant life. See for example U.S. Pat. No. 2,794,727. It is believed that propargyl bromide is a good replacement for methyl bromide. See K. C. Barrons, Farm Chemicals International, 2000 35-36.
A typical process for producing propargyl bromide involves low temperature initiation of reaction of propargyl alcohol and phosphorus tribromide in a liquid phase in the presence of base which usually is a tertiary amine such as triethylamine. While workable, undesirable amounts of waxy solids are often formed in the reaction mixture. Further, because the reaction is exothermic, the requirement for low temperatures (e.g., 5-6xc2x0 C. or below) at the start of the reaction adds refrigeration costs to the operation. Previous attempts to synthesize propargyl bromide from phosphorus tribromide and propargyl alcohol in the absence of a base yielded large amounts of undesired side products; see L. Henry, Chemische Berichte, 1873, 6, 728.
It would be of considerable advantage if a way could be found of eliminating or at least significantly reducing formation of waxy solids in the process. It would be particularly advantageous if this could be done while avoiding the need for expensive refrigeration in the operation. An additional advantage would be the accomplishment of these things with a concomitant decrease in the formation of undesirable side products.
This invention is deemed to enable achievement of the above advantages.
A feature of this invention is that propargyl bromide can be produced in good yield while minimizing the formation of undesired side products, such as 1,3-dibromopropene, 2,3-dibromopropene, and bromoallene. Surprisingly, this result can be achieved at higher addition temperatures than heretofore appreciated. And the process is economically advantageous, as a base such as pyridine or triethylamine is not required.
Provided by this invention is a process of producing propargyl bromide, which process comprises:
A) bringing together in a reaction zone under an inert atmosphere and in the absence of a base and in the presence of an inert diluent, a feed of phosphorus tribromide and a separate feed of propargyl alcohol thereby forming a reaction mixture;
B) while mechanically agitating the mixture being formed in A), maintaining the temperature of the mixture in the range of about 0xc2x0 C. to about 25xc2x0 C. to form a product mixture, and then
C) raising the temperature of the product mixture to a temperature in the range of about 40xc2x0 C. to about 60xc2x0 C. while stirring the product mixture for a ride period of at least about 2.5 hours.
Such process can be conducted as a batch process, as a semi-batch process, or as a continuous process.
Other embodiments and features of this invention will become still further apparent from the ensuing description and appended claims.
When conducting a process in accordance with this invention there are basically two ways of bringing the reactants together in the reaction zone. One way is to feed the propargyl alcohol to the reaction zone and then feed the PBr3 to the propargyl alcohol in the reaction zone. In such case, it matters not how or when the diluent is introduced into the reaction zone as long as at least sufficient diluent is present when the PBr3 and propargyl alcohol are coming together in the reaction zone to serve as a heat sink for the heat of reaction evolved. Thus it is convenient to introduce all of the diluent before starting the feed of the PBr3, or to introduce a substantial portion of the diluent before starting the feed of the PBr3 and to introduce additional diluent along with the feed of PBr3, either as a preformed solution of PBr3 in the diluent, or as separate but concurrent feeds of PBr3 and of diluent.
The second way of bringing the reactants together in the reaction zone is to feed propargyl alcohol and PBr3 separately but concurrently into the reaction zone. Here again it matters not how or when the diluent is introduced into the reaction zone as long as at least sufficient diluent is present to serve as a heat sink for the heat of reaction evolved when the PBr3 and propargyl alcohol are coming together in the reaction zone. For example, all of the diluent can be introduced into the reaction zone before starting the separate but concurrent feeds of the PBr3 and of the propargyl alcohol. Alternatively, a substantial portion of the diluent can be introduced into the reaction zone before starting the concurrent feeds and additional diluent can accompany either or both such concurrent feeds. In other words, additional diluent maybe introduced as a preformed solution of PBr3 in the diluent, as a preformed solution of propargyl alcohol in the diluent, and/or as a separate but concurrent feed with the concurrent feeds of PBr3 and of propargyl alcohol.
In any event, the point at which the propargyl alcohol and the phosphorus tribromide come into contact with each other is part of the reaction zone. Preferably, the reactants are concurrently fed into a reaction zone composed of at least one reactor in which all of the componentsxe2x80x94whether fed individually or in any subcombination(s)xe2x80x94all come together for the first time and in which the process is initiated and carried out.
When the propargyl alcohol and the phosphorus tribromide come into contact with each other, the mechanical agitation can be accomplished by the use of, for example,jet mixers or static mixers. Optionally, high shear mechanical agitation and/or high speed jet dip legs may also be used.
The use of the term xe2x80x9cconcurrentxe2x80x9d does not exclude the possibility of inconsequential interruptions taking place during the feeds. Nor does this term imply that the feeds must start at exactly the same moment in time. In the case of a co-feed process, the two feeds can be initiated with an interval of time between such initiation as long as the interval is sufficiently short as to cause no material adverse effect upon the overall process. Likewise in the case of a tri-feed or multi-feed operation, there maybe one or two different time intervals between or among the respective feeds, again provided that the time intervals are of sufficiently short duration to cause no material adverse effect upon the overall process.
The processes of this invention, whether performed in a batch mode, semi-batch (semi-continuous) mode, or continuous mode, are preferably conducted so that such things as the feeds, reaction, and maintenance of the desired temperature occur xe2x80x9ccontinuouslyxe2x80x9d during the reaction. However, it cannot be stressed strongly enough that one must not gain the impression that inconsequential interruption in one or more of such things cannot occur. Interruptions which do not materially affect the conduct of the process are not excluded from the scope of this invention. Whatever the terms used, the process should be conducted as one of ordinary skill in the art would carry out the processes after a thorough, unbiased reading of this entire disclosure and in keeping with the spirit of the invention gained from such a reading.
As is well known in the art, operation under an inert atmosphere requires the presence of an inert gas such as nitrogen, argon, or helium. This minimizes or excludes oxygen from the reaction zone. Nitrogen is a preferred inert gas in the practice of this invention. The inert gas can be introduced into the reaction zone by various means, such as sweeping the reaction zone with an inert gas prior to the introduction of the inert diluent, or passing the inert gas into the reaction zone during the process.
The inert diluent used is typically one or more (i) paraffinic hydrocarbons, (ii) cycloparaffinic hydrocarbons, or (iii) aromatic hydrocarbons or a mixture of any two or all three of (i), (ii) and (iii), but can be any inert liquid, i.e., a liquid which does not react with either the reactants or the products produced in the reaction in such a way as to prevent formation of propargyl bromide. Thus a diluent that solvates or complexes with a reactant or product of the reaction can be used provided that the formation of propargyl bromide is not prevented by its use; such a diluent is deemed to be inert within the meaning of this disclosure.
Because propargyl bromide is a high energy material that is sensitive to physical shock or impact, and that is also susceptible to rapid thermal decomposition upon exposure to high temperatures or fires, it is preferred to employ an inert solvent which forms an azeotrope with propargyl bromide, such as a paraffinic and/or cycloparaffinic hydrocarbon solvent that forms an azeotrope with propargyl bromide. By xe2x80x9cazeotropexe2x80x9d is meant a mixture that under temperature and pressure conditions encountered at any normal stage of the life-cycle of propargyl bromide, the propargyl bromide and a stabilizing amount of the hydrocarbon when in the liquid or vapor state remain together at all times. Also when propargyl bromide is to be used as a fumigant, especially as a soil fumigant, it is desirable that the diluent used in the process be an environmentally-acceptable azeotropic inert liquid solvent that remains with and protects the propargyl bromide against hazardous shock-induced or thermally-induced decomposition whether the propargyl bromide is in the liquid state or in the vapor state.
By xe2x80x9cenvironmentally-acceptablexe2x80x9d is meant that the inert liquid satisfies or, if not yet evaluated, will satisfy the requirements for listing as an xe2x80x9cinertxe2x80x9d or xe2x80x9cother ingredientsxe2x80x9d in categorized List 1, List 2, List 3, or List 4 of the Office of Pesticide Programs of the United States Environmental Protection Agency, such lists as updated Jun. 12, 2001. Such lists are incorporated herein by reference as if fully set forth herein, except that all substances on such lists which do not meet all criteria specified herein are excluded from such lists because they are incapable or unsuitable for use herein. The features of this paragraph are more fully described in commonly-owned copending application Ser. Nos. 10/118,290, filed Apr. 8,2002, and 10/126,260, filed Apr. 18, 2002.
Various azeotropic solvents can be used in the practice of this invention. Non-limiting examples include n-heptane, mixed heptane isomers, cyclohexane, methylcyclohexane, 2-methylhexane, 2,4-dimethylpentane, n-octane, isooctane, 2-methylheptane, 2,2-dimethylhexane, isopropyl alcohol, and a mixture of cyclohexane and isopropyl alcohol. A preferred solvent mixture is composed of a mixture of C7-9 hydrocarbons (e.g., Isopar E, ExxonMobil Chemical Corporation) in admixture with cyclohexane. A particularly preferred azeotropic solvent is a mixture composed primarily of C8 isoparaffinic hydrocarbons such as Isopar C (ExxonMobil Chemical Corporation).
Mechanically agitating the reaction mixture being formed minimizes localized heat buildup in the reaction zone, ensures good mixing, and might minimize formation of undesired side products. Without wishing to be bound by theory, it is believed that good agitation may be important for achieving a decrease in undesired side products. At higher temperatures, the use of agitation to prevent heat buildup becomes increasingly important. On the laboratory scale, agitation rates are normally in the range of about 15 rpm to about 20 rpm.
During the bringing together of the phosphorus tribromide and the propargyl alcohol, the temperature of the reaction mixture being formed needs to be controlled, because of the exothermicity of the reaction of phosphorus tribromide and propargyl alcohol, and because of the thermal sensitivity of the propargyl bromide being formed. The temperature is maintained in the range of about 0xc2x0 C. to about 25xc2x0 C., and preferably in the range of from about 5xc2x0 C. to about 20xc2x0 C. Preferably, the temperature is continuously maintained in the desired range. While it is possible to perform the addition at temperatures above 25xc2x0 C., it is not recommended for safety reasons and because the yield of propargyl bromide decreases. For a description of the sensitivities of propargyl bromide to shock and to thermal decomposition, see D. R. Forshey et al., Fire Technology, 1969 5 100-111.
A molar excess of phosphorus tribromide can increase the yield of propargyl bromide. Thus, preferably, a stoichiometric excess of phosphorus tribromide relative to propargyl alcohol is used. This can be accomplished using either of the two ways described above of bringing the propargyl alcohol and the phosphorus tribromide together. When propargyl alcohol is fed to the reaction zone before phosphorus tribromide, phosphorus tribromide is fed until the desired excess has been added to the reaction zone. When propargyl alcohol and phosphorus tribromide are fed separately but concurrently to the reaction zone, the phosphorus tribromide is fed in an amount that is a molar excess relative to the amount of propargyl alcohol being fed to form the reaction mixture. The amount of molar excess of phosphorus tribromide is preferably in the range of about 3% to about 15% relative to propargyl alcohol. More preferably, the phosphorus tribromide is in the range of about 5% to about 10% molar excess relative to propargyl alcohol.
Raising of the temperature of the product mixture and subjection of the product mixture to the ride period can occur in the reaction zone where the product mixture was formed, or in a different location (e.g., another reactor or vessel). For the ride period, ride times are typically at least about 2.5 hours on the laboratory scale. During the ride time, the temperature is usually in the range of about 40xc2x0 C. to about 60xc2x0 C. Preferably, the temperature during the ride period is in the range of about 40xc2x0 C. to about 55xc2x0 C. Primarily for safety reasons, it is highly preferred to keep the ride period temperature no higher than about 50xc2x0 C.
The following Examples are presented to illustrate the practice of, and advantages made possible by, this invention. These Examples are not intended to limit, and should not be construed as limiting, the scope of this invention to the particular operations or conditions described therein.